As a method of preparation of a relief printing plate by forming a concavity and convexity on the surface, a so-called “analog plate making”, that is, a method in which a photosensitive elastomer composition or photosensitive resin composition is exposed to ultraviolet light through an original film to selectively cure the image area and the uncured area is removed with a developer is well known.
The photosensitive elastomer composition contains an elastomeric polymer, for example, a synthetic rubber, as a carrier and since the relief image formed therefrom is flexible, the printing plate is referred to as a flexographic printing plate.
The flexographic printing plate has an aptitude for water-based ink, alcohol ink and ester ink each using ink vehicle which does not corrode the rubber and non-solvent UV ink. Since the flexographic printing plate is flexible, it has a printing aptitude for a printing material having a large concavity and convexity on the surface or a packaging material having a low strength. However, since it is apt to be deformed by the application of stress, a printing pressure is necessary to be low. On the other hand, the photosensitive resin composition uses a plastic resin (plastic) as the carrier. The relief printing plate obtained is hard and is referred to as a resin anastatic printing plate (letter press) which is distinguished from the flexographic printing plate. Commercially available resin anastatic printing plates include a water development type and an alcohol development type and contain a water-soluble resin and an alcohol-soluble resin, respectively. Ink mainly used therefor is oil-based ink using ink vehicle which does not corrode the resin and non-solvent UV ink. Since the resin anastatic printing plate is hard, a high printing pressure can be applied and clear and sharp printing can be performed by supplying a large amount of ink.
Since the analog plate making requires an original film using a silver salt material in many cases, the time and cost for producing the original film are necessary. Further, in order to develop the original film, a chemical treatment is required and treatment of the waste liquid of development is also needed, the analog plate making is disadvantageous in view of environmental health.
As a means for solving the problems according to the analog plate making, a flexographic printing plate precursor and the resin anastatic printing plate precursor having a laser-sensitive mask layer element capable of forming an image mask on site (in situ) provided on the photosensitive elastomer layer and the photosensitive resin layer, respectively, are proposed (see, for example, Japanese Patent 2773847 and JP-A-9-171247 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). According to a plate making method of the printing plate precursor, the printing plate precursor is subjected to laser irradiation based on image data controlled by digital devices to form an image mask from the mask layer element on site, then similar to the analog plate making, the printing plate precursor is exposed to ultraviolet light through the image mask and either the photosensitive elastomer layer or the photosensitive resin layer and the image mask are removed by development. The plate making method is referred to as a “mask CTP system” in the field of flexographic printing plate or resin anastatic printing plate. Although the mask CTP system resolves the problems relating to the process of producing the original film described above, the problems on the treatment of waste liquid resulting from the development of the photosensitive elastomer layer or the photosensitive resin layer still remain. Further, in case of the flexographic printing plate, since a chlorine solvent, for example, trichloroethylene is used in the development in many instances, the system is also disadvantageous in view of working health.
As a means for solving the problems of the development process and development waste liquid, a so-called “heat development system”, that is, a method in which the photosensitive elastomer layer is heated and the uncured portion is removed by softening is proposed (see, for example, JP-A-2002-357907). Since the system does not use a developer, it is favorable in view of working environment and the development waste can be subjected to incineration disposal without a particular segregation process. However, since the developing speed of the heat development system is extremely low in comparison with that of the solvent development system, other problems in that the working efficiency is poor and in that a complicated and costly development apparatus is required.
As another means for solving the problems of the development process and development waste liquid, a so-called “direct engraving CTP system”, that is, a direct engraving plate making method with laser is often proposed. The direct engraving CTP system is literally a method of making a concavity and convexity to form a relief by engraving with laser and is advantageous in that unlike the formation of relief using an original film, the formation of relief can be freely controlled. For instance, it is possible that a position where an outline character is reproduced on a printed material is deeply engraved and that in a part where minute halftone dots are reproduced, the halftone dots are engraved to form shoulders in order to prevent collapse of the halftone dots due to the printing pressure.
In Japanese Patent 2846954, JP-A-11-338139 and JP-A-11-170718, a laser-engravable flexographic printing plate precursor and a flexographic printing plate obtained by laser engraving are described. In Japanese Patent 2846954, JP-A-11-338139 and JP-A-11-170718, a monomer is mixed with an elastomeric rubber as a binder and the mixture is cured by a heat polymerization mechanism or photopolymerization mechanism and then is subjected to laser engraving to obtain a flexographic printing plate.
As a problem of the direct engraving CTP system, it is illustrated that the seed of laser engraving is low. This is because in the direct engraving CTP system, at least a thickness of 100 μm is necessary to engrave in view of the feature of directly forming the relief, in contrast with the mask CTP system wherein a thickness of the mask layer element which is an object to be ablated is approximately from 1 to 10 μm. Thus, some proposals intended to improve the laser engraving sensitivity have been made.
For instance, a flexographic printing plate precursor for laser engraving containing an elastomer foam is proposed (JP-A-2000-318330). Although the improvement in laser engraving sensitivity is intended by using the foam having low density, a problem arises in that because of using the material of low density, the strength for a printing plate is insufficient and printing durability is severely impaired.
Also, a flexographic printing plate precursor for laser engraving including microspheres containing hydrocarbon gas encapsulated is proposed (U.S. Patent Publication No. 2003/180636). The improvement in laser engraving sensitivity is intended by the system in which the gas in the microspheres expands with heat generated by laser to destroy the graving material. However, a problem arises in that the strength for a printing plate is apt to be insufficient because of the material system including the gas. Further, since the gas has a property of easy expansion with heat in comparison with a solid, even when microspheres having a high heat deformation initiation temperature are used, change in the volume due to fluctuation of the outer temperature can not be avoided and thus it is not suitable to use as a printing plate which is required stability of the accuracy of thickness.
Further, a flexographic printing plate precursor for laser engraving containing a polymer filler having a ceiling temperature of less than 600K is proposed (JP-A-2000-168253). Although the improvement in laser engraving sensitivity is intended by adding the polymer filler having a low depolymerization temperature, the use of such a polymer filler makes a concavity and convexity on the surface of printing plate precursor which seriously influences printing quality.
It is known that by using polylactic acid in combination with an oxide of alkaline earth metal, thermal degradability and biodegradability of polylactic acid increase (H. Nishida et al, Polymer, 45, 1197 (2004), and JP-A-10-273582). However, the construction of the resin composition for laser engraving according to the invention, specifically, the resin composition for laser engraving comprising a combination of a binder polymer containing a polyester and an oxy compound of at least one of specific metals and metalloids described hereinafter has not been disclosed in the prior art.